Geographical Topic Discovery and Comparison

Zhijun Yin, Liangliang Cao, Jiawei Han,

Chengxiang Zhai, Thomas Huang

UIUC

To appear in WWW’11

Presenter: Jeff Huang

Outline • Motivation

• Problem Formulation

• Solution Sketch

• Experiments

• Q/A

3/21/2011 2

Motivation • GPS records are popular on the Web

o Advanced cameras with GPS receivers could record GPS

locations when the photos were taken.

o Some applications including Google Earth and Flickr provide

interfaces for users to specify a location on the world map.

o People can record their locations by GPS functions in their smart

phones.

3/21/2011 3

Motivation (Cont.) • Examples of GPS-associated documents

o Flickr: geo-tagged photos

o Twitter: tweets from iPhone

3/21/2011 4

Motivation (Cont.) • What can we do?

o By analyzing the geographical distribution of food and

festivals, we can compare the cultural differences around

the world.

o We can also explore the hot topics regarding the candidates in presidential election in different places.

o We can compare the popularity of specific products in

different regions and help make the marketing strategy.

3/21/2011 5

Motivation (Cont.)

6

• Discovering different topics of interests that are

coherent in geographical regions.

• Comparing several topics across different

geographical locations.

• Geographical topic discovery and comparison

3/21/2011

Problem Formulation • A GPS-associated document is a text document

associated with a GPS location.

• A geographical topic is a spatially coherent theme.

In other words, the words that are often close in

space are clustered in a topic.

• An example of geographical topics o Given a collection of geo-tagged photos related to festival with

tags and locations in Flickr, the desired geographical topics are

the festivals in different areas, such as Cherry Blossom Festival in

Washington DC and South by Southwest Festival in Austin, etc.

3/21/2011 7

Problem Formulation (Cont.) • Given a collection of GPS-associated documents

o Discover the geographical topics

o Compare the topics in different geographical

locations.

3/21/2011 8

Problem Formulation (Cont.) • An example of geographical topic discovery and

comparison

o Given a collection of geo-tagged photos related to food

with tags and locations in Flickr, we would like to discover

the geographical topics, i.e., what people eat in different areas. After we discover the food preferences, we would

like to compare the food preference distributions in

different geographical locations.

3/21/2011 9

Problem Formulation (Cont.) • A topic distribution in geographical location is the

distribution of the topics given a specific location.

o Formally, p(z|l) is the probability of topic z given location l

= (x, y) where x is longitude and y is latitude.

3/21/2011 10

Geographical Topic

Discovery and Comparison • Given a collection of GPS-associated documents D

and the number of topics K, we would like to

discover K geographical topics, i.e., where

Z is the topic set and a geographical topic z is

represented by a word distribution

s.t. .

• Along with the discovered geographical topics, we

also would like to know the topic distribution in

different geographical locations for topic

comparison, i.e., p(z|l) for all z Z in location l.

3/21/2011 11

Zzz }{

Vwz zwp )}|({ 1)|( Vwzwp

Solution • Location-Driven Model (LDM)

• Text-Driven Model (TDM)

• Location-Text Joint Model (Latent Geographical

Topic Analysis (LGTA))

3/21/2011 12

Location-Driven Model (LDM) • LDM

o Clustering based on document locations

o One location clustering is a topic

o Generate topic description for each cluster

• Disadvantage o No text guidance

o It is possible that there is no spatial cluster patterns. A

geographical topic may be from several different areas

and these areas may not be close to each other.

• In landscape dataset, mountains exists in different areas and

these areas are not close to each other

3/21/2011 13

Text-Driven Model (TDM) • Discover the geographical topics using topic modeling

o Topic modeling with network regularization [Mei et al. WWW’08]

o Regularization based on the closeness in location between

documents

• Disadvantage

o How to define the document closeness w(u, v)?

o How to have the topic distribution of locations p(z|l)?

3/21/2011 14

LOCATION-TEXT JOINT MODEL • Main Insight: Construct a model to encode the

spatial structure of words

o The words that are close in space are likely to be clustered

into the same geographical topic.

• Assume there are a set of regions. The topics are

generated from regions instead of documents.

o If two words are close to each other in space, they are

more likely to belong to the same region.

o If two words are from the same region, they are more likely

to be clustered into the same topic.

3/21/2011 15

Latent Geographical Topic Analysis (LGTA)

3/21/2011 16

region

importance

p(z|d) p(w|z)

location shape

• Combine text and location information

• Adapts the region discovery process according to

the dataset.

Parameter Estimation • EM algorithm

• Iterations: o Geo-clustering (region

discovery) is based on both location and topic

information.

o Topic modeling is based on

the text and region information.

3/21/2011 17

Data Set • Flickr images with GPS locations

o Flickr API supports search criteria including tag, time, GPS

range, etc.

3/21/2011 18

Compared Methods • LDM: Location-driven model

• TDM: Text-driven model

• GeoFolk [Sizov WSDM’10]: o A topic modeling method that uses both text and spatial information.

o Model each region as an isolated topic

o Assume the geographical distribution of each topic is Gaussian

• LGTA: Latent Geographical Topic Analysis

3/21/2011 19

Topic Discovery Comparison • Festival dataset

o Topics related to South By Southwest Festival

3/21/2011 20

Topic Discovery Comparison • Activity dataset

3/21/2011 21

Topic Discovery Comparison

• Landscape dataset

3/21/2011 22

coast desert mountain

LDM

TDM

GeoFolk

LGTA

• Average distance of word distributions of all

pairs of topics by KL-divergence

3/21/2011 23

Topic Quality Qualitative Comparison

Topic Quality Qualitative Comparison

• Text Perplexity

3/21/2011 24

Topic Quality Qualitative Comparison

• Location/Text Perplexity

3/21/2011 25

3/21/2011 26

Geographical Topic Comparison

27 3/21/2011

• Complicated model and parameter estimation

• How to set the number of regions and the number

of topics?

• How about estimating geographical locations for

images that are without geo information? o Generating representative photos for the landmarks